Seismic data acquisition system

ABSTRACT

A seismic survey system having remote acquisition modules (RAMs) for acquiring seismic signals and communicating with a central recording system (CRU) via a network of cables, other RAMs, and line tap units (LTUs), arranged in a matrix of receiver lines and base lines. Each RAM cyclically converts analog signal values to digital, forming data packets. Interrogation commands emanating from the CRU and relayed with strategic delays by intervening LTUs and RAMs are received by the RAM. Each command causes the RAM to transmit a data packet. Strategic delays are set such that the transmission capacity of the line is best utilized. Power and frequency of transmission are selectable by the CRU to optimize performance. Cables contain multiple communication pairs. The network path between the RAM and the CRU is established from the CRU and altered in event of malfunction. All types of network elements are interconnectable. Recorded samples are synchronous.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a Division of pending U.S. patent application Ser.No. 11/220,244 filed Sep. 6, 2005, now U.S. Pat. No. 7,209,407. Saidapplication Ser. No. 11/220,244 is a Division of U.S. patent applicationSer. No. 10/162,243 filed Jun. 4, 2002, and now U.S. Pat. No. 6,977,867issued Dec. 20, 2005. Said application Ser. No. 10/162,243 was filedwith claim to the benefit of the Jun. 5, 2001 filing date of U.S.Provisional Patent Application Ser. No. 60/296,089

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to seismic survey acquisition equipment.In particular, the invention relates to seismic survey equipmentassembly combinations, survey data management strategies, operatingsoftware for carrying out the management strategies, the logistics ofequipment deployment, and operation of equipment.

2. Description of Related Art

In principle, a seismic survey represents a voluminous data setcontaining detailed information that may be analyzed to describe theearth's layered geology as indicated by seismic wave reflections fromacoustic impedance discontinuities at the layer interfaces. The analysisis influenced by elastic wave propagation velocities respective to thedifferences in strata density or elasticity. A seismic event such as iscaused by the ignition of buried explosives in a shallow borehole or bya vibratory mechanism placed at the earth's surface is launched into theearth at a precisely known location and time. Seismic wave reflectionsof this man-made seismic event are detected by a multiplicity oftransducers characterized in the art as geophones. The geophones aredistributed in an orderly grid over the area of interest. The locationof each geophone array is precisely mapped relative to the location ofthe seismic event. As the seismic wave from the timed event travels outfrom the source, reflections from that original seismic wave arereturned to the surface where they are detected by the geophones. Thegeophones respond to the receipt of a wave with a corresponding, analogelectrical signal. These analog signals are received by data acquisitionmodules that digitize the analog signal stream for retransmission to acentral recording unit. Among the significant data digitized by a dataacquisition module may be the amplitude or strength of the reflectedwave and the exact time lapse between the moment the event occurred andthe moment an analog value of the geophone array is translated to adigital value.

In a single survey, there may be thousands of geophone signal sources.Consequently, the data flow must be orderly and organized. For example,the data acquisition modules transmit geophone signal values in digitaldata groupings called packets. Each packet contains a predeterminednumber of digital data bits representing, among other things, thedigital value of the analog signal amplitude at the time that a seismicwave or increment thereof was translated to the digital value. Theacquisition modules are programmed to transmit a data packet respectiveto a set of geophone channels at a predetermined bit rate. The variabledata in a data packet represents an instantaneous snapshot of the analogsignal flow from each geophone channel. There may be numerous individualgeophone units transmitting respective analog signals to the dataacquisition module on the same geophone signal channel.

Managing an orderly flow of this massive quantity of data to a centralrecording unit, often in a field survey truck, requires a plurality ofdigital signal processing devices. The data acquisition modules convertthe geophone analog data to digital data and transmit the digital datapackets along receiver lines or radio transmission channels. There maybe numerous data acquisition modules transmitting respective datapackets along a single receiver line. Among the functions of each dataacquisition module is data packet transmission timing respective to theflow of data packets from other data acquisition modules transmittingrespective data packets along the same receiver line. Typically, two ormore receiver lines connect with base line units that further coordinatethe data packet flow of numerous additional base line units into a basetransmission line for receipt by a central recording unit.

Seismic surveying is often carried out under extremely inhospitableconditions of heat or cold, tropics or arctic, land or sea, desert orswamp. Regardless of the environment, the geophones must be positionedprecisely relative to the seismic source event. Necessarily, manualplacement of the geophones is normally required.

One of the many challenges facing seismic ground crews using cableconnected systems is the initial decision of cable configuration(s).Data demands by geologists and investors are not always predictable.Seismic contractors must try to choose cable configurations thatminimize weight for their workers in the field while keeping the numberof line connectors to a minimum. However, prior art seismic systems areinflexibly designed as an integrated unit. If a remote data acquisitionmodule is designed to operate in an 8-channel mode, a prior art systemcannot readily be reconfigured to operate in a 6-channel modenotwithstanding that a particular survey task may be especially suitedto the 6-channel mode. Prior art data acquisition modules aremanufactured for a typical configuration with a fixed bit transmissionrates and power settings that may not be adjusted. Consequently, bittransmission rates and power of transmission are mandated which areoptimum only for a single type of equipment configuration.

Prior art systems rely upon interrogation commands from the centralcontrol module which are synchronously transmitted to the remote dataacquisition modules, relying solely on the central system clock tocontrol times of sampling.

An object of the present invention, therefore, is to assist a fieldobserver to maximize an efficient use of the recording resourcesavailable to him for any particular task. Another object of theinvention is to provide the greatest possible quantity of data of thehighest possible quality for a given equipment configuration.

Another object of the present invention is a seismic system that mayhave its bit transmission rate tuned to optimize application of theavailable cable and other equipment to the seismic task objectives.

A further object of the present invention is to utilize deliberatelyasynchronous sampling of data at the remote units to increase efficiencyof utilization of the network components.

Also an object of the present invention is the provision of aconfigurable seismic telemetry network having multiple data transmissionpaths available by remote selection. A further object of the inventionis a remotely actuated termination point for data interrogation signals.

An additional object of the present invention is a seismic telemetrynetwork in which all data carriers may function at the same bittransmission rate.

Still another object of the invention is a seismic telemetry network inwhich data transmission base lines may be operated at transmission ratesgreater or less than those of receiver lines when advantageous to thesurvey geometry. Prior art provides base lines operating at fixedtransmission rates higher than the receiver line transmission rates.These prior art systems do not provide means to easily vary the bit rateof base line transmission to take advantage of differing requirements ofseismic surveys or to match base line bit rate to the bit rate of thereceiver line transmissions.

Other objects of the invention include an extension of receiver linetake-out distances by optimizing data signal strength. Transmissionelectrical power influences the distance over which reliable telemetrycan occur with higher power required for longer distances. Prior artdoes not provide ability to vary power as may be required to optimizecommunication for variable transmission distances over different cables,such as may be used within a project or on projects with differingrequirements. Power conservation is an important consideration inprolonging battery life in a distributed seismic data acquisitionsystem. Conservation of battery power in the distributed telemetry unitsby limiting transmission power to a minimum required for reliablecommunication is an object of this invention.

Receiver line take-out distances are also enhanced by an increase indata transmission efficiency. By an optimization of communication for agiven receiver line take-out distance, the weight of equipment for agiven system configuration is reduced.

Also an object of the invention is an increase in the time density ofdata transmission by minimizing wasted time between data packets.

A further object of the invention is to increase the efficiency of datatelemetry by excluding information from the data packet that wouldidentify the signal processing unit that originated the data and itstime of creation (which reduces the amount of data that is to betransmitted) and to use the position of the data packet within the datastream to implicitly communicate data packet identity.

The capacity and option to selectively split the data-reporting route ofportions of receiver lines is also an object of the present invention.

Another object of the invention is to provide network elements that areinterconnectable and able to perform multiple functions therebymaximizing flexibility and efficiency of equipment utilization.

SUMMARY OF THE INVENTION

The foregoing objects of the invention and others not specificallystated above will be apparent to those of ordinary skill in the art fromthe following detailed description of the invention. Each RemoteAcquisition Module (RAM) of the present invention is controlled by aCentral Recording Unit (CRU) for cyclically converting analog seismicamplitude values to digital values. The digital values respective to acycle are combined with other information as a digital data packet.Alternate RAMs in a receiver line transmit respective data packets alongone of two communication lines to a Line Tap Unit (LTU) for transmissionto a CRU. Data packets are transmitted from respective RAMs on commandfrom interrogation signals. The interrogation signals are initiated froma CRU and retransmitted from the LTUs to the nearest RAM, whichimmediately begins transmitting data assembled since the previoustransmission cycle. The interrogation signal, however, is delayed fromretransmission to the next RAM until the data packet of the first RAMmay be accommodated by the segment of communication conduit between thefirst RAM and the LTU. Interrogation signal retransmission is timed toreceive the first data packet from the next RAM as transmission of thelast data packet of the first RAM is completed. This pattern is repeatedfor all RAMs in a receiver line.

Transmission bit rate is adjusted to an appropriate value between about6 to 12 megabits per second (mbps), for example, to accommodate thenumber of data packets to be transmitted along a given receiver line ina transmission cycle. Also considered in the transmission bit rateselection are the properties and physical characteristics of the cablebetween the RAMs in a receiver line series. However, the RAMs and baseline units have 1 to 2 megabytes of data memory, for example, toaccommodate a surplus of data generation. The data storage may besufficient to store and entire sequence of shot data for latertransmission. Alternatively, the data storage may be used to allow datatransmission at a slower rate than the rate of data creation during theperiod of recording.

Transmission signal power is also adjusted to an appropriate value toboth provide reliable communication between adjacent RAMs (and LTUs) andto minimize power consumption, thus prolonging battery life in thedistributed units.

Base line transmission rate may be selected to be the same as thereceiver line transmission rate to match capacity of the two types ofcommunication, or base line transmission rate may be set higher or lowerthan receiver line transmission rate to take advantage ofcharacteristics of the survey such as differing in-line and cross-linespacing and/or differing in-line and cross-line data volumerequirements.

Collaterally, the system has the capacity to logically link all receiverlines with selectable communication conduit whereby a receiver line maybe terminated where desired by commands issued from the centralrecording unit. The data packets from the RAMs isolated along onereceiver line may be transmitted to another base line along anotherreceiver line or they may be left unused.

Another characteristic of the system is RAM channel flexibility wherebyany number of channels may be accommodated, up to the maximum capacityof a RAM. Consequently, the RAMs are not limited to a fixedcommunication scheme having a specific number of signal channels perRAM. RAMs constructed according to the present embodiment of theinvention may be connected with 2, 4, 6, and 8 channel cable, forexample.

The system further provides a flexible, multi-path network forconnection of RAMs to the CRU. Universal cable connectors allow receiverline, base line and jumper cable to be connected to any type of devicein the network, including RAMs, LTUs and the CRU. RAMs may be used asrepeaters. Receiver line cable may be used as base line cable withreduced number of conduits. The system operator manipulates the networkusing a graphic user interface with a substantially true-scale map ofthe survey area that shows the location of physical obstacles and allseismic survey equipment items and their network connections. Systemsoftware guides the operator in optimizing the equipment and networkconfiguration, while overcoming physical barriers and sporadic equipmentfailures.

Bi-directionality of the RAMs and multi-directionality of LTUs, combinedwith looping of cables and logical breaks in receiver line cables,together with the inter-connectibility of cables and modules, providesadaptability not available in prior art.

Because the RAMs and LTUs are configurable from the CRU, necessarychanges can be made to the network configuration without physical visitsto remote line equipment modules when changing circumstances requirealteration of the configuration. Multiple communication conduits withinreceiver-line and base-line cables provide opportunities to optimize useof transmission capacity and avoid shut down in case of disruption ofsome of the conduits. The multiple conduit base-line cable design isexploited in base-line splitting and rejoining to bypass obstacles anddistribute capacity on both sides of obstacles.

The invention includes a new method of operating a seismic network whichis deliberately asynchronous to allow more efficient telemetry ofseismic data to the CRU and which utilizes independent clocks in theRAMs to more efficiently control the timing of samples. The asynchronoussampling is converted to synchronous sampling through a novel processingmethod effected by the RAMs and the CRU.

This processing method enables accurate and precise timing of seismicsignal amplitude values and also overcomes the inaccuracy of the clocksin the individual RAMs. This method predicts time delays inherent in thenetwork and measures RAM clock drift. Utilizing a highly accurate CRUclock and the sampled amplitude values, estimates of the amplitudevalues at the correct times are determined. This feature of theinvention allows implementation of continuous recording as opposed toconventional intermittent recording as is useful and necessary in lateststate-of-the-art land and marine seismic systems.

Another unique feature of the invention includes a definition of theseismic network by specifying the location and status of all systemelements including RAMs, LTUs and the CRU. The network definitionfurther includes a specification of the exact order of transmission ofdata packets by all active elements in the network, enabling an implicitdetermination of the identity of the RAM that originated a data packetand its time of creation. This method of implicit conveyance ofinformation reduces the amount of data to be physically transmitted andimproves network efficiency. A method of compensating for missing orsurplus data packets is also provided to make the method of implicitidentification of data packets more practicable.

BRIEF DESCRIPTION OF THE DRAWINGS

Other features and advantages of the invention will be recognized andunderstood by those of skill in the art from reading the followingdescription of the preferred embodiments and referring to theaccompanying drawings wherein like reference characters designate likeor similar elements throughout the several figures of the drawings andwherein:

FIG. 1 a is a half-plan schematic of the invention as deployed for a 3Dsurvey;

FIG. 1 b is a half-plan schematic of the invention as deployed for a 3Dsurvey;

FIG. 2 is a detailed schematic of communication conduit between a pairof RAMs and geophones connected to the RAMs;

FIG. 3 is a cross-sectional view of an 8 channel receiver line cable anduniversal connector;

FIG. 4 is a cross-sectional view of an 8 channel base line cable anduniversal connector;

FIG. 5 is a functional schematic of a remote acquisition module (RAM);

FIG. 6 is a functional schematic of an analog-to-digital conversionmodule;

FIG. 7 is a functional schematic of a RAM communication module.

FIG. 8 is a functional schematic of a base line unit (BLU);

FIG. 9 is a functional schematic of a line tap unit (LTU);

FIG. 10 is a functional schematic of a central recording unit (CRU);

FIG. 11 is a functional schematic of a communications module for a CRU;

FIG. 12 is a functional schematic of a base line splitter;

FIG. 13 is a data table and corresponding graph correlating cable lengthand signal transmission rate for a comparison of two types of cable;

FIG. 14 is a tabulation of possible survey layout parameters availablewith the invention;

FIG. 15 is a diagram showing typical equipment layout and signal flowrouting for the invention;

FIG. 16 is the diagram of FIG. 15 but with a revised signal flow routingdue to a receiver line break;

FIG. 17 is a schematic illustration of a typical base line splitterapplication;

FIG. 18 is a typical map display of a seismic equipment field layoutused to overcome physical barriers, superimposed upon a topographicalmap;

FIG. 19 is a graph of interrogate command time skew; and,

FIG. 20 is a seismic wave graph illustrating seismic signal skew andsignal amplitude interpolation;

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 schematically illustrates a model seismic survey matrix accordingto the invention wherein geophones are distributed over the terrain ofinterest in an orderly manner of period and spacing. For this example,the geophones are aligned in four rows, T1, T2, T3 and T4. Row T3 isextended discontinuously across a physical obstacle such as a river orhighway. Distributed along each of the geophone rows are three (forexample) RAMs, 10. Construction of a RAM 10 will be described more fullywith respect to FIGS. 5, 6 and 7.

The RAMs are connected by two Receiver Line cables 12 respective to an“A” side and a “B” side of each RAM. See FIG. 5. As shown incross-section by FIG. 3, a receiver line cable 12 comprises four pairsof geophone channel conduits 32 and two pairs of communication conduits30 and 31, surrounding a stress carrying core element 28. The six pairsof receiver line conduit are aligned within an insulation annulus 24 andencased by a shield jacket 26. The receiver line cable is terminated atboth ends with a universal cable connector 39. This cable connectorallows connection of the receiver line cable to any RAM 10, LTU 14, BLU38 or to the CRU 18 as shown in FIG. 1. The connector pins include onepair 135 for communication conduit 30, a second pair 136 forcommunication conduit 31, four pairs 137 for geophone channel conduits,and two unused pairs 138. The unused pairs are retained to allow use ofa universal cable connector 39 for all types of cable in the system,including receiver line cable, base line cable and jumper cable types.

Referring to FIG. 1, the two receiver line cable sections respective toRAMs R-1/RAM 1 and R-1/RAM 2 in row T1 are mutually joined by aback-to-back connection 36. The same is true for the receiver linecables between R-2/RAM 2 and R-2/RAM 3 in row T2. Row T4 includes twoback-to-back connectors 36. The back-to-back connectors 36 providecontinuity between communication conduits 30 and 31 of connectedreceiver line cables 12 but not for the geophone channel conduits 32.Each of the four geophone channel conduits 32 in a single cable sectionrespectively connects to only one RAM. Hence, each RAM receives up toeight geophone channels in this preferred embodiment example.

It is common industry practice for each geophone channel 32 to beconnected with a plurality of geophones. Each of the geophonesrespective to a given channel 32 has a predetermined position relativeto the seismic disturbance location whereby those commonly connectedgeophones all receive substantially the same subsurface reflectionsignal thereby (through summation) reinforcing the signal strength butreceive substantially different seismic noise, thereby attenuating noisewhen summed.

Usually, not always, the geophone signals through the channels 32 areanalog: analog-to-digital conversion being performed by the RAM as willsubsequently be described more fully. However, A/D conversion bydedicated circuitry in individual geophone units is possible and isadvantageous under certain circumstances.

Again referring to FIG. 1, LTUs 14 ₁, 14 ₂ and 14 ₃ join the rows T1, T2and T3 to a base line cable 16. The LTUs will be described more fullywith respect to FIG. 9.

The base line cable 16 shown in cross-section by FIG. 4 comprises eightcommunication conduit pairs 34 ₁₋₈ within an insulation annulus 24 andshield jacket 26. At the core of the assembly may be a stress carryingcore 28. A universal cable connector 39 terminates both ends of asection of base line cable, allowing connection to any module in thesystem. Connector pins 147 for communication conduits 34 are shown. Theuniversal connector 39 is physically identical to the connector used inthe receiver line and jumper cable types allowing fullinter-connectability of all equipment in the system.

Shown in FIG. 1 is a section of base line cable 16 joining LTUs 14 ₄ and14 ₅. The use of base line cable 16 instead of receiver line cable 12 toconnect RAMs that are on the same logical receiver line, as in thisexample, illustrates one aspect of the inter-connectability andadaptability of the system.

The eight communication conduits 34 ₁₋₈ (FIG. 4) connect the geophonefield matrix to a CRU 18 (FIG. 1) that is often carried in a vehicle formobility. Depending on the data processing capacity of the CRU 18, oneor more base lines 16 may serve a CRU 18. There are eight communicationconduits in the base line cable 16 and two communication conduits ineach receiver line cable 12. Geophone data will be reported to the CRU18 along the four receiver lines R-1 through R-4. Two of the eightcommunication conduits of the base line are made available to eachactive receiver line ensuring a one-to-one correspondence betweenreceiver line and base line conduits that are utilized.

Specifically, receiver line R-1 serves RAMs R-1/RAM 1 and R-1/RAM 2.Data from geophone channels 1-8 connected to RAM R-1/RAM 1 is initiallyprocessed by that RAM and transmitted along receiver line communicationconduit 30 ₁ to base line communication conduit 34 ₅. The data producedby geophone channels 9-16 of row T1 is processed by RAM R-1/RAM 2 andtransmitted along receiver line communication conduit 31 ₁ to base linecommunication conduit 34 ₁.

Receiver line R-2 serves R-2/RAM 1, R-2/RAM 2, R-2/RAM 3 in row T2 andR-2/RAM 4 in row T3. The communication conduit 30 ₂ and 31 ₂ respectiveto the cable 12 end sections for rows T2 and T3 are linked by a jumpercable 17. The jumper cable is a cable that may contain only twocommunication conduits and no geophone channel conduits. It may be usedto connect the ends of two receiver lines to form a loop. The data ofgeophone channels 9-16 in row T2 is transmitted by R-2/RAM2 and channels17-24 in row T3 is transmitted by R-2/RAM 3 along receiver linecommunication conduit 30 ₂ to base line communication conduit 34 ₆. Thedata of geophone channels 25-32 in row T3 is transmitted by R-2/RAM 4along receiver line communication conduit 31 ₂ to base linecommunication conduit 34 ₂. Also, geophone data from channels 1-8 of rowT2 is transmitted by R-2/RAM 1 along communication conduit 31 ₂ to baseline communication conduit 34 ₂.

Receiver line R-3 serves only geophones 1-8 in row T3 that are signalprocessed by R-3/RAM 1. The data is transmitted along receiver linecommunication conduit 30 ₃ to base line communication conduit 34 ₇.

Receiver line R-4 serves R-4/RAM 1 in row T1 and R-4/RAM 2 in row T3. Inrow T4, receiver line R-4 also serves R-4/RAM 3, R-4/RAM 4 and R-4/RAM5. Geophone channels 25-32 in row T4 are connected to R-4/RAM 4 for datatransmission along receiver line communication conduit 30 ₄ to base linecommunication conduit 34 ₈.

Receiver line communication conduit 31 ₄ receives the data of geophonechannels 1-8 in row T1, channels 17-24 in row T4 and channels 33-40 inrow T4 for transmission to the CRU along base line communication conduit34 ₄. Receiver line communication conduit 30 ₄ receives the data ofgeophone channels 9-16 in row T3 and channels 25-32 in row T4 fortransmission to the CRU along base line communication conduit 34 ₈.

The invention embodiment of FIG. 2 illustrates two receiver lines R-1and R-2 connected to a base line 16. The communication conduits 30 ₁ and31 ₁ of receiver line R-1 connect R-1/RAM 1, R-1/RAM 2 and R-1/RAM 3 tobase line communication conduits 34 ₁ and 34 ₅, respectively. Thecommunication conduits 30 ₂ and 31 ₂ of receiver line R-2 connectR-2/RAM 1, R-2/RAM 2 and R-2/RAM 3 to base line communication conduits34 ₂ and 34 ₆, respectively.

The RAMs 10, the LTUs 14, and the CRU 18 communicate by several types ofdigital data packets. The CRU 18 uses “Commands” to communicate with theline equipment comprising the RAMs 10, LTUs 14, BLUs 38 and repeaters.The line equipment sends Line Data back to the CRU. Each piece ofequipment in the matrix system knows its orientation relative to theCRU. RAMs and LTUs recognize only Commands on their CRU side and LineData on their line side.

Each RAM and LTU inherently has a logical “Command Side” and a “LineSide”. There is no physical difference between the two sides, and eitherphysical side may play either functional role. Definitively, however,the Command Side is the side closer to the CRU, normally, with apossible exception in the case where both physical sides are reachableby direct path from the CRU (requires looping of Receiver Lines by useof jumper cable 17). In the preferred embodiment, in a multipathenvironment as in FIG. 2, the Command Side of each device is determinedby the CRU under the control of the operator. The CRU may switch thesides of a particular RAM or LTU as the survey progresses. This would bedesirable, for example, in response to a communication failure in aparticular cable segment. Another benefit of this ability to configurethe directionality of the RAMs and LTUs is that when the CRU is moved toanother location during the course of the survey, these modules arereadily adapted to the new network configuration by the operator(without the necessity of a physical visit to the site of every RAM andLTU to be reconfigured, as in prior art).

In FIG. 2 the jumper cable 17 allows RAMs to communicate with the CRUfrom either side, and thus with a simple re-assignment of Command Side,an otherwise stranded RAM can be accessed by the CRU. The CRU controlsthe assignment of Command Side at system initialization by sending a“power-up” voltage to the device.

A digital data packet includes 204 bits per packet. Of this total, 8data bits are reserved for a packet identification header, 192 bits areavailable for data use, and 4 bits are reserved for a data integritycheck (checksum).

Commands may comprise, for example, of 32 bits of data for instructingone (or all) line equipment module to perform a given task. For example,the software may instruct a particular LTU to “power off” all RAMs onits “B” side. In another case, the software program may instruct allRAMs to switch into a low-power mode. Typically, the data bit structureof a Command data packet devotes the first 5 bits in the sequence toidentification of a packet type e.g. Command, Interrogate Command orLine data. The sixth and seventh bits in a Command packet identify thetype of device (RAM, LTU, etc.) to which the Command is being sent. Theeighth bit in the Command preamble is a “global” bit that defines whichdevices are to act on the Command. One setting of the global bitaddresses all devices of the selected type. Another setting incorporatesthe 16 following bits to specifically designate which devices are to acton the Command (Addressed Command). The last 8 bits in a Command packetdefine the Command being sent. When an LTU receives a command from thetruck, it forwards the command simultaneously in three directions: outthe “A” side, the “B” side and the “Line side” (unless the command isthe special case of an Interrogate Command, which is treateddifferently). As each RAM on the spread receives a Command, it decides(based on the preamble and address bits) whether or not to act upon it,then sends it to the next device on the line.

Interrogate Commands are a special type of Command consisting of only 8bits. The Interrogate Commands tell all devices to transmit Line databack to the CRU if primed to do so by previous Commands. In identifyingan Interrogate Command, a device looks at only the first five bits ofdata and ignores the rest. Upon receiving an Interrogate Command, an LTUpasses it to the RAMs on its “A” side and on its “B” sidesimultaneously, then begins transmitting toward the CRU the prior timesample “A” and “B” data which it has stored in memory. When the priorsample data has been transmitted for the “A” and “B” sides, the LTU,having purposely delayed sending the Interrogate Command out the “Line”side to minimize the gap in transmission of data towards the CRU, beginstransmitting newly received “Line” side data (from the current sample)toward the CRU.

If an LTU does not receive enough responses within the programmed lengthof time, it inserts simulated data for the missing RAMs. If the LTUreceives too many responses, it ignores those over the defined number.This method allows the CRU to identify the origin of data packetswithout resorting to use of explicit identification bits within the datapacket. Once finished with the “A” side, the LTU repeats the process onthe “B” side. Thereafter, the LTU sends the Interrogate Command out the“Line Side” side.

The LTU must transmit data toward the CRU in this order “A”, “B” and“Line” sides. The order transmitted is the same order as would haveoccurred if it had actually interrogated the “A” side, the “B” side andthe “Line” sides in turn. This strict adherence to the correct orderingof data packets for transmission toward the CRU is necessary forreducing data packet size through omission of identifying information,which improves efficiency of the telemetry.

A RAM or LTU may be used in Repeater Mode. In this mode its function ismerely to receive Commands from the CRU and transmit them on the “Line”side to the next RAM or LTU. In Repeater Mode the RAM or LTU alsoreceives data from the “Line” side and decodes and re-transmits the datatoward the CRU.

When an active RAM (activated by previous Commands) receives anInterrogate Command, it begins sending its data towards the CRU. Justbefore finishing transmitting its data packet, (e.g. at a timecalculated to minimize the time gap in transmission) the RAM passes the8-bit Interrogate Command to the next RAM or LTU on the line.

Line Data packets consist of 204 data bits, for example. These packetsinclude either analog-to-digital (e.g. geophone pulse or geophone noise)or status (e.g. battery voltage, serial number, etc.) information sentby line equipment to the recording system. The first 8 bits of a LineData packet are the preamble. Bits 1-5 identify the block of informationas data from the line as described previously. The next 3 bits identifywhat type of information is contained in the packet and how it wasoriginated. For example, information may be real or simulated shot data,or device status.

The Data Word portion of a Line Data packet is 192 bits long and mayinclude either shot data (24 bits from each of a RAM's eight channels)or status information. The remaining four bits of a Line Data packet arethe Checksum Count. Before a RAM sends data to the recording system, itcounts the number of “high” bits (or “1 s”) in the Data Word and writesthe total here in binary format. The RAM counts in cycles of 16 (from 0to 15), repeating the cycle until it finishes counting all high bits inthe Data Word. For example, if a total of 20 bits were set “high” in theData Word, the RAM would count to 15 then repeat the cycle, counting 16as 0, 17 as 1, 18 as 2, 19 as 3 and 20 as 4. The Checksum Count in thiscase would be 4 (written as “0-1-0-0” in binary format).

After a RAM sends Line Data towards the CRU, each device along the wayverifies it. When a device receives Line Data, it counts the high bitsin the Data Word and compares that number with the data packet'sChecksum Count. If these numbers do not match, the device notes the factthat it detected a transmission problem. The device then sends the datatowards the CRU and waits for more data from the line or the CRU.

After collecting data, the system polls all devices on the line in orderto determine which devices detected transmission problems and where toplace error flags on the CRU monitor display, for example.

The construction of a RAM 10, as is shown schematically by FIG. 5,comprises a communication module 40 and an analog-to-digital conversionmodule 42. The communication module 40 is supported by a clock circuit44 and a Central Processing Unit (CPU) 46. The CPU includes a randomaccess memory circuit 48. The communication module is energized by apower supply circuit 45 that manages the power demands upon an internalbattery 47 and an external battery 49.

The schematic of analog-to-digital module 42 is shown more expansivelyby FIG. 6 to include, for each analog signal channel 32, a line surgeisolator 50 for limiting stray voltage surges; an analog signalamplifier 52; and an analog-to-digital converter 54. Eachanalog-to-digital converter 54 transmits, upon receipt of aninterrogation signal (called an Interrogate Command) from thecommunication module 40 (FIG. 5), its current geophone signal value tothe communication module 40 for integration into a respective datapacket.

The communication module 40 of a RAM is schematically represented byFIG. 7 to comprise a line surge isolator 56 to limit voltage surgescarried by the communication conduits 30 and 31. Digital values of thegeophone signals are received from the analog-to-digital converter 42.The delivery of the digital signals is coordinated by the CPU 46 toencode a data packet onto one or the other of the communication conduits30 or 31. Of the two communication conduits 30 and 31 in a receiver line12, one is selected to receive the data packet transmission. The othercommunication conduit is decoded and retransmitted by a repeater circuitin the controller 60. Generally, each communication conduit 30 or 31 islogically connected for data packet input from alternate RAM units alonga single receiver line. With respect to FIG. 2, for example,communication conduit 30 ₁ may be connected to receive data packets fromR-1/RAM 1 and R-1/RAM 3 whereas R-1/RAM 2 may report data packets alongcommunication conduit 31 ₁.

Under the software program control of the CPU 46, FIG. 5, and paced bythe clock circuit 44, the controller 60 (FIG. 7) receives the digitalsignal values from the analog-to-digital conversion module 42 andcombines that data with other header and with the checksum data tocreate a data packet. The seismic sampling rate is programmable fromabout 0.125 samples per ms to about 4 ms/sample, for example. Amplitudedata are stored in the RAM's memory until an Interrogate Command isreceived, after which it transmits the amplitude data in the form ofdata packets along the Receiver Line toward the CRU.

Along the receiver lines 12, signal streams comprising a series of datapackets are redirected into base line 16 signal streams by either LTUs14 or BLUs 38. The only difference between the two signal transmissionunits is an expanded data memory capacity for the BLUs 38. Both LTUs andBLUs potentially have signal processing capability.

With respect to the FIG. 9 schematic of an LTU 14, for example, thepreferred embodiment of the invention comprises communication conduitsfor a pair of receiver lines 12 _(a) and 12 _(b) and communicationconduits for a pair of base lines 16 _(a) and 16 _(b). Each of theseports is served by a remotely controlled line isolator circuit 64. Inthe normal operational mode, communication conduits 30 _(a) and 31 _(a)respective to receiver line 12 _(a) and communication conduits 30 _(b)and 31 _(b) respective to receiver line 12 _(b), are connected to thecommunication module 70. Similar to the RAM 10, the communication module70 of LTU 14 is directed by a CPU 72 and paced by a clock circuit 74.The CPU 72 memory capacity is expanded by random access memory 76. Aunit power distribution circuit 66 is supplied by internal batteries 68and/or external batteries 67.

The BLU 38 of FIG. 8 is substantially the same as an LTU 14 of FIG. 9except for bulk data storage capacity 78. A BLU may be used in place ofan LTU, but not necessarily vice versa. In instances where the bulk datastorage of the BLU is not required, the terms “LTU” 14 and “BLU” 38 areused interchangeably in this description of the preferred embodimentsand in the following claims.

The preferred embodiment of the CRU 18 is represented by FIG. 10 toinclude communication conduits for two base lines 16 that are served byrespective communication modules 80. The communication modules 80 arepaced by a clock 82 and externally powered by a source 84 such as abattery or generator. A power management circuit 86 includes bothfiltering and distribution. A CPU 88 controls the communication modules80. The CPU 88 is functionally supported by a random access memory 85and a bulk data storage circuit 87. The entire system is manuallyinterfaced by a keyboard 90, a monitor 92, a mouse 94, a plotter 96 anda printer 98.

The communication modules 80 for the CRU 18 are illustratedschematically by FIG. 11 to include line isolators 100 ₁₋₈ for each ofthe eight communication conduits 34 ₁₋₈ and a data controller 102.

There are several distinctive characteristics of the software programsthat control the invention operation. These distinctive characteristicscooperate to overcome several obstacles or inefficiencies inherent inprior art systems. One of these inefficiencies is an occurrence of largetime lapses between data packets resulting in a reduction in the amountof line equipment that can be accessed in a given time period. Anotherinefficiency arises from the complex relationship between (1) data cablelength, (2) data transmission bit rate and (3) data generation rate.

To address the prior art inefficiency of data rate transmission and toreduce the interval between data packets, the operational procedure ofthe invention includes a signal protocol by which the digital datapackets are assembled and queued for transmission from the numerous RAMsto the CRU 18. This procedure generally includes transmission of anInterrogate Command from the CRU to the LTUs 14. The LTUs relay theInterrogate Command on toward the RAM units along each of the receiverline communication conduits 30 and 31. Respective to the pair ofcommunication conduits 30 and 31 in a single receiver line 12, the twoInterrogate Commands are independently timed. They may or may not besimultaneously emitted. Although both of the communication conduits 30and 31 in a single receiver line 12 are connected to each RAM in therespective receiver line, the response each RAM will make to theconnection is normally different.

Referring to FIG. 2, an Interrogate Command A₀ originates from the CRU18 and is carried along communication conduit 34 ₁ of base line 16 toLTU 14 ₁, for example. The LTU 14 ₁ relays the Interrogate Command A₀along conduit 30 ₁ to R-1/RAM 1 Upon receipt, the R-1/RAM 1 beginsimmediately to sequentially transmit along the communication conduit 30₁, back to the LTU 14 ₁, the data packet containing the data of allgeophone system channels 32 (FIG. 5) connected to R-1/RAM 1.Significantly, the signal A₀ is not carried further along communicationconduit 30 ₁ than R-1/RAM 1. When signal A₀ is received by R-1/RAM 1, atiming delay is initiated by the RAM communication module 40 for therelay transmission of Interrogate Command A₁ along communication conduit30 ₁ from R-1/RAM 1 to R-1/RAM 3 via the repeater circuitry in R-1/RAM2. The length of this time delay is variable as a function of numeroussystem and project parameters. In particular, the time delay is moststrongly influenced by the number of geophone system channels connectedto a particular RAM (i.e. 4, 6 or 8), the cable type and length, thenumber of repeater RAMs and the transmission bit rate between the RAMs.The design philosophy of the retransmission delay of Interrogate CommandA₀ is to coordinate transmission of the last data packet from R-1/RAM 1with arrival of the first data packet from R-1/RAM 3 at R-1/RAM 1 and tominimize the inter-packet stream gap between the successive signalstreams. Although the Interrogate Command A₁ is received by R-1/RAM 2,the signal is merely repeated on to R-1/RAM 3.

When the Interrogate Command A₀ is received by R-1/RAM 1, transmissionof the data packets respective to the geophone system channels reportingto R-1/RAM1 (up to 8, for example) begins immediately. However,execution of the data packet signal requires a finite time period. Aportion of this finite time period is the delay interval for the relaytransmission of Interrogate Command A₁ by R-1/RAM 1. While the datapacket from R-1/RAM 1 is being transmitted back to the LTU 14 ₁,Interrogate Command A₁ advances to R-1/RAM 3 to initiate a correspondingdata packet transmission from that RAM. Immediately, transmission of theR-1/RAM 3 data packets begins along the segment of communication conduit30 ₁ between R-1/RAM 3 and R-1/RAM 1 that has carried InterrogateCommand A₁. The origination of Interrogate Command A₁ is timed to makethe first elements of the data packet from R-1/RAM 3 arrive at R-1/RAM 1just after the last of the R-1/RAM 1 data packet is transmitted.

An Interrogate Command B₀ transmitted from the CRU 18 independently ofInterrogate Command A₀ is relayed by LTU 14 along line communicationconduit 31 ₁ to R-1/RAM 1. Upon receipt of the Interrogation Command B₀,R-1/RAM 1 merely relays the signal on to R-1/RAM 2. R-1/RAM 2 beginstransmission of a respective data packet to the LTU 14 ₁ along thesegment of communication conduit 31 ₁ between R-1/RAM 2 and R-1/RAM 1.Upon receipt of the data packet, R-1/RAM 1 merely repeats the datapacket signals to the LTU 14 ₁.

The Interrogate Command delay at each of the RAMs is not a fixed valuebut is potentially variable for each RAM depending on the number ofanalog channels reporting to a respective RAM, the number of repeaterRAMs between the active RAMs and other factors affecting transmissiontime. Although the preferred embodiment of the invention provides for 8geophone system channels 32 to each RAM, the respective CPU 46 may beprogrammed to accommodate any number of channels less than 8, also.Moreover, there is no rule of nature that sets the maximum number ofanalog channels at 8. This is simply a matter of equipment design andengineering practicalities.

It should also be noted that the communication conduit 30 or 31 for aparticular RAM may be changed from one to the other. Such a step may berequired in the event of a broken connection or continuity in anintended communication conduit. However, in the event of such a change,the Interrogate Command delay time at the affected RAM may be altered.

Of especial note is a logical break capability of each RAM to beprogrammed for the termination rather than re-transmission to the nextRAM of an Interrogate Command. This capability allows the receiver linesto be looped and thus have cable connections to two LTUs 14.Functionally, however, in a given programmed configuration, each RAMwill operate with only one pair of communication conduits 30/31respective to a single, designated, LTU 14. In one example, asrepresented by FIG. 1, the continuity of geophone row T3 is interruptedbetween RAMs R-3/RAM 1 and R-2/RAM 4 by an insurmountable obstacle suchas a river or sheer cliff. Consequently, the Interrogate Command frombase line communication conduit 34 ₃ that would normally be transmittedto R-2/RAM 4 from LTU 14 ₃ is, instead, terminated by the LTU.Cooperatively, the Interrogate Command from LTU 14 ₂ that would normallybe terminated at R-2/RAM 3 is transmitted further via jumper cable 17 toR-2/RAM 4 in geophone row T3.

In the similar example shown in FIG. 2, the obstacle is represented bythe logical break line P-P across conduits 30 ₂ and 31 ₂ between R-2/RAM2 and R-2/RAM 3. Interrogate commands C₀ and D₀ from the CRU 18 arerelayed by LTU 14 ₂. Interrogate Command C₀ is received by R-2/RAM 1 anddelayed for retransmission to R-2/RAM 3 as Interrogate Command C₁.Because of a logical break command to R-2/RAM 2, Interrogate Command C₁is not issued. Meanwhile, Interrogate Command D₁ is relayed throughR-2/RAM 1 to R-2/RAM 2 for the R-2/RAM 2 geophone data. However, noretransmission signal D₁ is issued by R-2/RAM 2. The R-2/RAM 3 geophonedata is reported along conduit 30 ₁ via jumper cable 17 in response to adelay of Interrogate Command A₂ from R-1/RAM 3.

Although this result may obviously be accomplished by a physicaldisconnection of the communication conduits along the line P-P betweenR-2/RAM 2 and R-2/RAM 3, the need for such reporting reassignment maynot always be apparent at the time the RAMs are distributed. Moreover,certain RAMs may fail after distribution and require replacement, repairor omission. With the present invention, the options of omissions andrevised connections may be exercised from the CRU 18 as compared to theprior art options of repair or replacement that require a physicalreturn to the respective RAM locations.

The logical break capacity of the invention may be accomplished bydirect Commands (originated by the CRU) to the CPUs 46 respective to theRAMs. The CPUs 46 program the respective RAMs to cause them toselectively prevent the retransmission of Interrogate Command

By strictly defining the sequencing of data packets based on the networkconfiguration, the position of any data packet within the sequence maybe used to determine which RAM created that data packet. And because thedata packet sent in response to one Interrogate Command contains datasamples that were created proximate the time of the InterrogateCommand's arrival at the creating RAM, the time the data packet wascreated need not be explicitly stated within the data packet. The timeof creation is implicitly knowable by its position within the overalldata stream arising from the Interrogate Command

Thus, both the RAM of origin and the time of creation of any data packetcan be implicitly determined. This reduces the amount of data that mustbe explicitly written within the data packet. Therefore the total amountof data that is transmitted is reduced accordingly. This aids in theoptimization of seismic telemetry and makes the system more efficientand cost effective.

The multiple base line communication conduits, e.g. conduits 34 ₁through 34 ₈ and their respective receiver lines are each made toindependently follow the methods as described above for sequencing datapackets by the actions of the base line units and RAMs. Thus multipledata trains, one per base line communication conduit, existsimultaneously and may be operated in parallel, optimizing totaltransmission capacity.

Data packet integrity along communication conduits is affected bytransmission rate, transmission power, cable type and cable length. Ascable length increases so does the attenuation. Attenuation is greateras the transmission rate increases. To optimize the transmitted signaldefinition, the transmission bit rate and transmission power must betuned for the length of the communication conduit.

Data bit definition relates to the ability of the receiving instrumentto distinguish a data bit in the received signal continuum. Due totransmission line losses, data bit definition will decay over the lengthof the transmission line. At some point along the line length, the databit pulse that was transmitted has decayed beyond distinction fromrandom noise anomalies. Using a lower transmission bit rate, thedistance may be extended over which reliable communication may occur.Also, using greater power in transmission may extend this transmissiondistance.

The ability to control the power of transmission is a feature of thepreferred embodiment of this invention. The control is exercised fromthe CRU and determines the power level of transmission used by the RAMsand LTUs. The power level is increased as required for greater distancesof transmission and decreased for lesser distances. As different cablelengths and types may be used on one project, there may be differenttransmission power settings invoked for different RAMs within thenetwork, and the power level used by RAMs may differ from that used forLTUs. Power of signal transmission may be set differently for forwardtransmission toward the CRU and reverse transmission (away from theCRU). Power settings depend on transmission characteristics of thecommunication conduits of the cables, length being a primarycharacteristic, but other characteristics such as nature of theconductors also influences the power required and hence the optimumsetting. It is generally beneficial to conserve power by only usingsufficient power to ensure reliable communication but not excessivepower. This prolongs battery life in the remotely distributed RAMs andLTUs.

Determination of optimum power settings is done experimentally fordifferent types and lengths of cables and the CRU is programmed to usethese settings for the given cable, in the preferred embodiment. Powersettings are controllable independently of frequency of transmission.However, optimum power settings will be different for differentfrequencies of transmission, hence the CRU is programmed to recognizedifferent optimum settings for different frequencies of transmission, aswell as for different types and lengths of communication conduits.

By conserving battery power, productivity and cost effectiveness of thesystem are enhanced over that available from prior art.

The graph and associated table of FIG. 13 illustrate the operation ofthe present interrogation signal strategies as described above with twocables of differing conductor size and construction. This FIG. 13 graphplots the relation of transmission bit rate and cable length at thelimits of signal definition. To be noted from this comparison is theinfluence that cable construction has upon data transmission capacity.

For example, a 28 AWG conductor of construction “A” will transmitreliably discernable data over a cable length of 288 meters at 7.5 mbitsper second. Comparatively, a 26 AWG conductor of construction “B” maytransmit reliable data over a cable length of 342 meters at the sametransmission rate; a 54 meter extension that represents a 15% advantage.

The advantages of the invention are further illustrated by the tabulateddata of FIG. 14. Here, the capacity of the system is organized into 3groups respective to the number of geophone channels connected to eachRAM in an array. Specifically, the data of Group I corresponds to anequipment distribution matrix that connects 8 geophone analog channels32 to a single RAM. The Group II data corresponds to an equipment matrixhaving 6 geophone analog channels 32 connected to a single RAM. GroupIII data corresponds to a 4-channel connection.

Referring to the schematic of FIG. 1, the TO/Cable (takeouts percable)_column of the FIG. 14 table shows the preferred maximum number ofanalog geophone channel connections to a receiver line cable. The TOInterval (takeout interval) is the distance, in meters, between adjacentanalog connections along a cable length. The Weight column, is, inpounds, the weight of a corresponding cable of the tabulated length. TheDistance/RAM column is the spacial distance, in meters, between adjacentRAMs in a receiver line. The Cable Length column is, in meters, thelength of a corresponding cable.

The 8 columns of data respective to 8 Sampling Frequency values (i.e.Interrogation Frequency), 500 Hz, 400 Hz, etc., correspond to themaximum number of analog channels 32 that may be connected to a singlereceiver line of the tabulated length. The XMIT Rate column correspondsto the transmission bit rate_charging the respective receiver line. Aspecific number of analog channels 32 per receiver line listed by FIG.14 relates to the corresponding Sampling Frequency column and XMIT Raterow.

FIG. 1 depicts a typical land 3D seismic survey with receiver lines andbase lines that are perpendicular to the receiver lines. In some typesof 3D surveys the distance between receiver lines may be significantlyless than the distance between RAMs along the receiver lines. In thissituation it is advantageous in terms of optimizing base line telemetryto be able to select a higher transmission bit rate_than the rateselected to optimize receiver line telemetry, because the cable segmentsconnecting LTUs may be much shorter than the cable segments connectingRAMs. The CRU therefore, elects to use an appropriate higher rate oftransmission for the base lines, setting it independently from thereceiver line transmission rate. By using a higher transmission rate thebase line capacity is increased and more channels may be accommodated onone base line communication conduit. Using a lower transmission rate onthe receiver line communication conduit may be advantageous inparticular survey projects because it allows a greater distance betweenRAMs and hence fewer total RAMs to cover a given area.

Thus, in the preferred embodiment, the transmission rate of the baseline may be set to be higher, lower or the same as the receiver linetransmission rate. The system sets the transmission rates to be usedunder the control of the operator at the CRU and the CRU programs eachdevice in the network accordingly.

Seismic surveys have spatial and temporal sampling requirements that area function of the local geology, geophysical objectives, seismic noiseand signal characteristics and other factors. Sampling densityrequirements in time and space are both affected and in a similarmanner. Seismic surveys that have very shallow geologic targetsgenerally have the potential to retain signals at relatively highfrequency, e.g. 250 Hz. However to successfully image the shallowtargets at up to 250 Hz requires relatively dense spatial sampling aswell as dense time sampling. Conversely, deep geologic targets have thepotential to retain only lower frequency signal, e.g. up to 50 Hz.Imaging deep targets thus requires less dense time sampling (to defineup to 50 Hz) but beneficially also requires less dense spatial sampling.

As an example, a first seismic survey targeting very shallow geologichorizons may require very dense time sampling at a high sampling rate of500 Hz (to preserve with fidelity 250 Hz signal). To maintain reliablesignal definition, a short separation distance between adjacent RAMs isappropriate. From the table of FIG. 14, an extreme layout would connect1984 analog channels in a single receiver line to one side of an LTU.Cooperatively, the signal transmission rate (XMIT Rate) should be set atabout 16.25 Mbits per second. These analog channels could have maximumtake-out intervals of 17 meters along a maximum single cable length of136 meters. Only one cable would span between adjacent RAMs which arealso separated by a maximum of 136 meters. At each take-out point, thecable channel is broken and a geophone set is connected to an analogconduit line from the take-out point. A single analog conduit is brokentwice and reports in opposite directions to respective RAMs whereby eachRAM in the array is connected to 8 analog channels.

In the preceding example, although only 1984 channels may be connectedalong the receiver line to one side of the LTU, another 1984 channelsmay be connected along an extension of the receiver line if it isconnected to the opposite side of the LTU. Thus the operator may inpractice utilize double the number of channels per receiver line withrespect to the number of channels shown in the table, if he follows thispractice.

A subsequent survey targeting deep geologic layers with the sameequipment may require a very sparse sub-surface sampling that isdistributed over a large area. Long distances between geophone groupsand accordingly wide spacing between RAMs may be appropriate for such asurvey. Referring to FIG. 14, by adjusting the RAM sampling rate toabout 100 Hz and setting a transmission rate of about 3.5 mbits persecond; this low density survey could accommodate 416 analog channelsper receiver line (or 932 if receiver lines are connected on both sidesof the LTU). The RAMs could be spaced along the line at 528 meterintervals and connected to receive only 4 analog channels per RAM.Geophone take-out intervals along the data cable in this case may be amaximum of about 132 meters.

Thus, the adjustable sampling rate and signal transmission rate of thepresent invention, along with variability in the number of channels perRAM, allows optimization of the equipment investment for varying surveyrequirements. A variable bit rate translates directly into operationaland logistical advantages in the field. The transmit power controlfeature of the present invention is one more tool the user has to makedata transmission more robust under varying survey conditions, whileoptimizing power consumption. Data packet transmission control minimizesthe time gaps between data packet groups according to the cable type andlengths used in the network. This benefit provides the survey crew withclose to 100% time utilization of the cable with extra time availablefor more channels to be added to the line resulting in highercommunication conduit limits.

In the preferred embodiment of the system, the CRU 18 software isprogrammed to understand the 3-dimensional earth's surface and thelocation of geographic features, both natural and man-made, as well asthe location and operating status of all items of the seismic dataacquisition equipment. The CRU software understands the configurationand interconnections of the network of RAMs, receiver-line cable, LTUs,base-line cables and the CRU. The system operator is provided asubstantially true-scale map view of all of this information asexemplified in FIG. 18. The network connections may be established andmodified at any time by the operator or automatically by the software atthe request of the operator. In this way the desired subset of the totalset of deployed RAMs may be made active to record and transmit seismicdata when required to do so by the operator. Standard computer toolsincluding keyboard, mouse, touchpad and touch screen may be provided astools to the operator to assist him to manipulate the network to achievethe geophysical objectives. The operator may request the system softwareto optimize the network configuration to take best advantage of thecommunication capacity of the individual equipment items to reduce therequired transmission time to a minimum.

Looping of receiver lines (by joining ends of adjacent pairs of receiverlines using jumper cables 17) is a recommended practice in the preferredembodiment so that in event of failure of any RAM or breakage in thereceiver line cable, connection to the CRU may be re-established by useof the bi-directional communication capability of the RAM. The operatoris notified on the map screen of the failure and needs simply tore-direct the otherwise stranded RAMs to communicate in the oppositedirection to reach the CRU. This is done by re-positioning the logicalbreak in the receiver line. This is illustrated by the schematics ofFIGS. 15 and 16.

The originally expected data transmission routing is shown by FIG. 15wherein the data of RAMs 1-6 is transmitted along receiver lines R-1 toLTU 14 ₁. The data of RAMs 7-12 is transmitted along receiver line R-2to LTU 14 ₂. Although RAM 6 is physically connected to RAM 12 by loop17, the loop is off-line to the respective R-1 and R-2 InterrogateCommand transmissions from RAMs 6 and 12.

After the equipment array has been positioned and connected, unexpectedcircumstances cause a signal continuity interruption along receiver lineR-1 between RAM 3 and RAM 4 as shown by the X on FIG. 16. Responsivelythe operator of the present invention terminates the R-1 InterrogateCommand retransmission at RAM 3 (by insertion of a logical break),activates the R-2 Interrogate Command from RAM 12 and also terminatesthe R-2 Interrogate Command from RAM 4.

Failure of one of the two communication conduits in a receiver linecable during transmission will not result in loss of data because of twokey aspects of the system, (1) the storage of data in the memory of theRAM until the CRU confirms receipt of the data, and (2) the ability ofthe system to transmit all of the data over the remaining receiver linecommunication conduit. Although throughput capacity of the cable is cutin half, no data is lost.

Similarly, if a base line loses a portion of its communication conduits,for example due to physical damage during operation, all data may bedirected over the remaining conduits. The flexible network design allowsthis adaptability to unanticipated conditions.

Data storage is also available in the LTU (as it is with the RAM) whichallows saving of data while it awaits re-transmission to the CRU.

The capacity of the base line to communicate seismic data is provided byeight (8) independent communication conduits. In addition to providingredundancy useful in overcoming failure of some of the conduits asdescribed immediately above, this design facilitates the distribution ofbase-line capacity around both sides of physical obstacles. This isillustrated in FIG. 17. The base line needs to be connected to receiverlines on both sides of the obstacle. In prior art systems this wouldinevitably require the provision of two complete base lines distributedall the way from the CRU to the maximum extent of the area to becovered, an unnecessary burden in the preferred embodiment. Using thebase line splitter device 19, shown in FIG. 12, the capacity of thesingle base line from the CRU may be positioned on both sides of theobstacle. On the far side of the obstacle the base line may be re-joinedby use of another base-line splitter device 19. The eight selectedcommunication conduits may be spread evenly, four to each side, or inany combination totaling to eight. Conduits not selected at the splitare not connected and are unused around the obstacle. The base linecould be designed with a number of communication conduits different fromeight without changing the principles of this method, of course.

Instead of requiring two complete base lines from the CRU to the edge ofthe recording area, one suffices, except at the obstacle itself,resulting in a significant savings in labor and equipment. The basicconcept of providing base lines with sub-dividable capacity makes thisachievable. Prior art systems that use a high capacity base line cannotachieve this savings and are more subject to total loss of transmissioncapacity due to equipment failure.

The preferred embodiment also provides inter-connectability of networkdevices to make the total network more flexible and adaptable todifferent layout requirements. Either a base line cable or a receiverline cable may be connected to any port of the LTU. An LTU may beconnected to a receiver line between any pair of RAMs. Physical receiverlines may be connected at both ends to base lines, or to the same baseline at different LTUs. Base lines may be split and rejoined. Receiverlines may be used to carry base line telemetry.

FIG. 18 illustrates the benefits to seismic data acquisition operationsof the inter-connectability of the preferred embodiment. The operatorwith the guidance of the system software, uses the true scale map of thearea and the seismic equipment, and builds the network in the optimumway, given the nature of the obstructions.

In this example there are three kinds of physical obstacles thatobstruct the layout of the desired ideal grid of seismic receiver lines.There is a river running across the area, a highway inhibits access anda series of sandstone cliffs blocks access. The operator at the CRUviews the map as depicted in FIG. 18. This map changes as often asnecessary to depict the current equipment configuration. As the operatorconstructs the network he has the advantage of viewing the exactlocations of equipment items with respect to the physical features ofthe terrain. He also sees the operability status of equipment items, forexample whether a particular base line and the receiver lines with RAMsconnected to it are operating within specifications. He makes decisionswhich best utilize available equipment to build the network.

The operator has chosen to establish a separate base line south of thehighway to reduce safety concerns by limiting the number of cables andworkers on the highway. He has also chosen to establish a base linerunning north and to split it several times, one part staying below thecliffs, the other climbing the cliff at the easiest point, where itdivides again and again to take advantage of the topography.

At the NE corner of the area, the operator has chosen to use receiverline cable with RAMs used solely as repeaters and with no geophonesconnected to these RAMs. Here the receiver line cable has been used tocarry the base line telemetry and therefore acts as a base line withonly two communication conduits. An LTU at the end of this section ofcable joins a receiver line with the RAMs at the NE extremity of thearea. This illustrates the dual roles the RAMs may serve, i.e. as purerepeaters to overcome distance limitations, and as data acquisitiondevices for the geophone arrays. Also, the ability of the receiver linecable to substitute for base line cable, although with reduced number ofcommunication conduits, is another feature which increases systemflexibility and hence productivity. Prior art systems do not have thesecapabilities.

Jumper cables 17 are used to connect segments of receiver line cable tocreate loops at the ends of pairs of receiver lines. This allowsextension of receiver lines but also may provide alternate transmissionpaths that can be used to overcome cable breaks and failure of one ofthe RAMs in the pair of receiver lines.

Thus the operator with the aid of the map view and layout tools providedby the software, devises the most practical and cost-effective way toacquire the seismic data. The flexibility of the network improves easeand safety of deployment, but also improves productivity afterdeployment, as the multiple paths available from each RAM to the CRUallow continued production without need for re-deployment in event ofequipment damage or failure.

FIG. 18 depicts the CRU and a typical RAM in the network that isseparated from the CRU and connected to it by a base-line cable with aseries of LTUs and a receiver line with several intervening RAMs. Adesirable objective of all seismic data acquisition systems is thatamplitude samples are recorded by all RAMs in the network that, ineffect are all taken at the exact same instant. It is not necessary,however, to actually sample the amplitudes simultaneously if a means isavailable to know the varied actual times of sampling respective to eachRAM and a means is provided to calculate the probable values ofamplitude at the ideal sample time. The preferred embodiment of theinvention incorporates unique means to accomplish the sampling objectivestated above.

The method of the invention recognizes that there are two categories oferrors that cause the time of an amplitude sample to differ from theintended ideal time. The first category of errors includes those causedby the successive delays in the network as the Interrogate Commandtravels from the CRU through the series of intervening network elementsto the RAM. The second category of error occurs within the RAM.

Transmission delays in the base line cable, LTUs, receiver line cable,and intervening RAMs all contribute to the first category of error.These delays may be either physically measured in the laboratory priorto the seismic survey and tabulated in CRU system software for each typeof network element, or in the case of deliberately-imposed delays ofInterrogate Command retransmission, may be computed by the systemsoftware. The CRU is programmed to simply sum up these predictabledelays for the given network configuration and thereby compute the totalpredictable transmission delay for each RAM in the network. Thispredicted value equates to the total delay between the time theInterrogate Command is sent from the CRU until the given RAM takes thecorresponding initial amplitude samples for its channels at thebeginning of the period of recording.

In the preferred embodiment, after the first sample of a seismic recordis taken, the RAM continues to take samples at increments of time equalto the programmed sample period, for example every 2 ms, according tothe RAM's own internal clock. The RAM's internal clock may be arelatively low-powered and drift-prone clock, such asTemperature-Compensated Crystal Oscillator (TCXO), with a drift such as2.5 parts per million (PPM). However the system master clock in the CRUis much more accurate and also consumes much more power. Typically itmight have a drift rate such as 0.02 PPM. The system master clock may beperiodically corrected using an external time source such as from a GPSclock.

Freeing the RAM from dependence upon receiving each and everyInterrogate Command from the CRU prior to taking the next sample hasadvantages in terms of system efficiency and in error prevention in caseof sporadic errors in transmission from the CRU to the RAM and is anovel feature of the invention.

As the RAM proceeds to take amplitude samples after the initial samplein the recording period, say every 2 ms according to its clock, thesamples increasingly may drift away from the intended sampling times dueto increasing buildup of error in its clock. The error may become sogreat as to invalidate and render useless the amplitude data when thelength of the recording period is great if the method of the preferredembodiment is not used. FIG. 19 illustrates the buildup of clock drifterror between the time of the initial sample and the time of a latersample.

This RAM clock drift can be monitored in the following way which is themethod of the preferred embodiment.

-   -   1. The RAM stores its clock times periodically on a        predetermined schedule of receipt of Interrogate Commands, e.g.        every 100 receipts, beginning with the first Interrogate Command        at the beginning of a period of recording.    -   2. At the end of the period of recording, or when requested by        the CRU, the RAM sends back its table of stored clock times to        the CRU.    -   3. The CRU, knowing the times on its internal clock that        correspond to the times in the table containing the RAM clock        times, and knowing the total predictable delay for the RAM,        constructs a drift curve for the RAM's clock consisting of        values of RAM clock time versus master clock time.

Any Interrogate Command which fails in transmission and is thus notreceived by the RAM will decrease its count by one and cause adiagnostic shift in the drift curve. Unless transmission errors arerampant, the method includes detection and correction of suchInterrogate Command transmission errors.

Using the drift curve and the total predicted delay for each channel ofeach RAM, the CRU computes the actual times at which each RAM took itsamplitude samples. FIG. 20 shows the two sets of times, the desiredtimes and the actual times, marked off against a representative analogseismic waveform. The actual samples provide a basis for estimation ofthe amplitudes at the intended sample times according to the masterclock. A simple regression or curve-fitting method may be used tocompute the estimated amplitude values at the intended times.Alternatively, more elaborate methods well-known in the art such as (sinX)/X or optimum least-mean-square-error (LSME) interpolation filteringmay be used. The CRU thus computes amplitude values for the idealintended time of sampling for each recorded channel, effectivelyachieving the objective.

Accounting for drift of the RAM clock may be of minimal importance ifthe duration of the recording period is short, for example, 10 sec. Forvery long periods of recording such as 300 sec or longer, it isessential, and therefore is invaluable in implementation of continuousor quasi-continuous recording required by methods such as VibroseisSlip-Sweep.

An average time error for a channel, computed over a time-window ofrelatively short duration, e.g. 10 sec, may be used to time-shift all ofthe amplitude samples within this window, if the amount of relativedrift of the RAM clock is insignificant within the window (e.g. <0.2ms).

The RAM clock drift being different for each of the many RAMs in therecording system dictates that the original amplitude samples fordifferent RAMs to be taken at differing times. In this respect therecording system in this invention is an asynchronous system rather thana synchronous system as in the prior art. Furthermore thedeliberately-imposed delays in transmission of Interrogate Commandscontribute to the asynchronous nature of the system (while at the sametime allowing maximization of data throughput along base lines andreceiver lines).

The novel method of correcting the time samples enables an asynchronoussystem to achieve the desired sampling which is in effect synchronous.Because the system is initially asynchronous it is able to achievenetwork and system efficiencies not possible with synchronous systems.

Although our invention has been described in terms of specifiedembodiments which are set forth in detail, it should be understood thatthis is by illustration only and that the invention is not necessarilylimited thereto. Alternative embodiments and operating techniques willbecome apparent to those of ordinary skill in the art in view of thepresent disclosure. Accordingly, modifications of the invention arecontemplated which may be made without departing from the spirit of theclaimed invention

1. A method of processing seismic data signals transmitted by aplurality of remote acquisition terminals along a common receiver linewhereby first data signal transmission from a first acquisition terminalis initiated upon receipt of a first interrogation signal, transmissionof a second interrogation signal from said first acquisition terminal toa second acquisition terminal is initiated at a time interval followingreceipt of said first interrogation signal by said first acquisitionterminal, second data signal transmission from said second acquisitionterminal being initiated upon receipt of said second interrogationsignal, said time interval being coordinated to minimize the time delaybetween said first and second data signals.
 2. A method of processingseismic data signals as described by claim 1 wherein said first datasignal is carried by the same conduit that carries said firstinterrogation signal.
 3. A method of processing seismic data signals asdescribed by claim 2 wherein said second data signal is carried by thesame conduit that carries said second interrogation signal.
 4. A methodof processing seismic data signals as described by claim 1 wherein firstand second remote acquisition terminals are separated by a predetermineddistance and each receive seismic signals from a predetermined pluralityof seismic transducer channels, said time interval being a function of atransmission bit rate respective to said first and second data signals.5. A method of processing seismic data signals as described by claim 1wherein said time interval is also a function of the number of seismictransducer channels connected to each remote acquisition terminal.
 6. Amethod of processing seismic data signals transmitted by a plurality ofremote acquisition terminals respective to a plurality of a commonreceiver lines, said receiver lines emanating from a common base linewhereby first data signal transmission from a first acquisition terminalalong a first receiver line is initiated upon receipt of a firstinterrogation signal, transmission of a second interrogation signal fromsaid first acquisition terminal to a second acquisition terminal alongsaid first receiver line is initiated at a time interval followingreceipt of said first interrogation signal by said first acquisitionterminal, second data signal transmission from said second acquisitionterminal being initiated upon receipt of said second interrogationsignal, said time interval being coordinated to minimize the time delaybetween said first and second data signals.
 7. A method of processingseismic data signals as described by claim 6 wherein a third dataacquisition terminal in said first receiver line transmits third datasignals to an acquisition terminal in a second receiver line.
 8. Amethod of processing seismic data signals as described by claim 7wherein said third data signals are transmitted in response to a thirdinterrogation signal from the acquisition terminal in said secondreceiver line.
 9. A method of processing seismic data signalstransmitted by a plurality of remote data acquisition terminals along acommon receiver line whereby first seismic data signal transmission froma first data acquisition terminal is initiated upon receipt of a firstinterrogation signal, transmission of a second interrogation signal fromsaid first data acquisition terminal to a second data acquisitionterminal is initiated at a first time interval following receipt of saidfirst interrogation signal by said first data acquisition terminal,second seismic data signal transmission from said second dataacquisition terminal being initiated upon receipt of said secondinterrogation signal, said first and second seismic data signals,respectively, having leading elements and trailing elements, said firsttime interval approximately being the time required to coordinate anarrival of the leading elements of said second seismic data signal atsaid first data acquisition terminal with the transmission of thetrailing elements of said first seismic data signal.
 10. A method ofprocessing seismic data signals as described by claim 9 wherein saidfirst seismic data signal is carried by the same conduit that carriessaid first interrogation signal.
 11. A method of processing seismic datasignals as described by claim 9 wherein said second seismic data signalis carried by the same conduit that carries said second interrogationsignal.